Generally, I've had lots of success with Calibre Companion, but I have a few (not all) PDFs which get corrupted in the "Send to device" process.

When I open the content server in Calibre, I can download the files into (e.g.) MoonReader and open it just fine, but when I send the PDFs to Calibre Companion wirelessly, the resulting file in unreadable in MoonReader, Adobe's PDF Reader, and a few other apps. They all describe the file as "damanged or corrupted".

I compared the file in Calibre_Companion on the device with the file in my local Calibre library (using WiFi File Browser to transfer it), and they are different sizes, and have different content (see below):

Generally, I've had lots of success with Calibre Companion, but I have a few (not all) PDFs which get corrupted in the "Send to device" process.

When I open the content server in Calibre, I can download the files into (e.g.) MoonReader and open it just fine, but when I send the PDFs to Calibre Companion wirelessly, the resulting file in unreadable in MoonReader, Adobe's PDF Reader, and a few other apps. They all describe the file as "damanged or corrupted".

What you are describing seems to be a calibre problem. The difference between the content server (which you can connect to with Calibre Companion also) and Send to device (which is used for transferring to any device including Calibre Companion) is that Send to device updates the PDF metadata when transferring to the device. So the file should be a different size when using Send to device versus using the content server, but it should work fine. The fact that it doesn't work indicates a problem and you should submit a bug report.

As a work around, until a solution is found, you can stop the PDF metadata update by going to Preferences - Plugins - Metadata writer plugins - select Set PDF metadata and click the Enable/Disable plugin button to disable the plugin and you should be fine.